1. Field of the Invention
This invention relates to a moving picture compression apparatus and a moving picture expansion apparatus suitable for a system for transmitting or storing moving picture signals, such as a digital television broadcasting equipment or a digital video disc.
2. Description of the Related Art
In a system for transmitting or storing digital moving picture signals, picture signals are encoded (compressed) by exploiting intra- or inter-frame correlation of moving picture signals for efficient utilization of a transmission channel or a storage medium. As encoding (compression) techniques for moving picture signals, there is known a compression system standardized by a research organization termed Moving Picture Experts Group (MPEG) for encoding moving pictures for storage.
In the method for compressing picture signals by exploiting the intra-frame correlation, orthogonal transform, such as discrete cosine transform (DCT), capable of concentrating coefficients for encoding, is predominantly employed.
In the method for compressing picture signals by exploiting the inter-frame correlation, so-called motion-compensated inter-frame prediction is predominantly employed. The principle of the motion-compensated inter-frame prediction is now explained by referring to FIG. 1. It is assumed that pictures P1 and P2 have been generated at time pints t1 and t2, respectively, with the picture P1 having been transmitted and with the picture P2 being about to be transmitted, as shown in FIG. 1. The picture P2 is split into plural blocks for each of which the amount of motion (motion vector) between it and the picture P1 is detected. The motion-compensated inter-frame prediction resides in finding a difference picture between a prediction picture and the block of the picture P2 and encoding the difference picture and the motion vector. The prediction picture corresponds to the picture P1 moved in translatory movement a distance equal to the motion vector.
FIGS. 2 and 3 shows a conventional moving picture compression device which takes advantage of the above-described intra- and inter-frame correlation, and the structure of a conventional moving picture expansion device, respectively.
The conventional moving picture compression device, shown in FIG. 2, compresses the input digital picture signal entering a picture input terminal 101 to output the compressed signal at a bitstream output terminal 109.
In the conventional moving picture compression device, shown in FIG. 2, the picture signals entering the picture input terminal 101 is routed to a motion vector detector 112 where the motion vector is calculated. The motion vector information as found by the motion vector detector 112 is sent to a motion compensation unit 110 which then motion compensates a picture stored in a frame memory 111, based on the motion vector, for formulating a prediction picture.
The digital picture signals entering the picture input terminal 101 are also routed to a difference calculation unit 102 which then calculates the difference between the picture signals entering the picture input terminal 101 and the prediction picture formulated by the motion compensation unit 110. The difference signal, thus found by the difference calculation unit 102, is routed to an orthogonal transform unit 103 for orthogonal transform. The signal orthogonal transformed by the orthogonal transform unit 103 is routed to a quantizer 104 where it is quantized for compression. The quantized data is routed to a multiplexer 108 where it is multiplexed with the motion vector information and outputted at a bitstream output terminal 109.
The data quantized by the quantizer 104 is also routed to a dequantizer 105 where it is dequantized and then inverse orthogonal transformed by an inverse orthogonal transform unit 106. This produces the same difference picture as that restored from the output bitstream. The signal of the difference picture and the signal of the prediction picture formulated by the motion compensation unit 110 are summed together by an adder 107 to produce picture signals which are entered to the frame memory 111 for the above-mentioned motion compensation.
On the other hand, the conventional picture expansion device shown in FIG. 3 expands a bitstream entering an input terminal 121 to output the expanded bitstream at a picture output terminal 126.
Referring to FIG. 3, the bitstream entering an bitstream input terminal 121 is sent to a motion vector separator 122 where the motion vector information is separated from the bitstream. This motion vector information is sent to a motion compensation unit 127 which then motion compensates a picture in the frame memory 128 for constructing a prediction picture.
The quantized data taken out of the bitstream by the motion vector separator 122 is routed to a dequantizer 123 for dequantization and thence supplied to an inverse orthogonal transform unit 124 for inverse orthogonal transform to generate a difference picture. The signals of the difference picture and those of the prediction picture produced by the motion compensation unit 127 are summed together by ah adder 125 to produce picture signals which are stored in a frame memory 128 while being outputted at the picture output terminal 126.
The moving picture compression device and moving picture expansion device, as described above, are occasionally connected in series to each other, as shown in FIG. 4. The compression and expansion devices, thus interconnected in tandem, as shown in FIG. 4, are equivalent to a device for repeatedly executing compression and expansion.
Specifically, the picture signals supplied to a picture input terminal 200 in FIG. 4 are compressed by a moving picture compression device 201 and outputted at a bitstream output terminal 202. This bitstream is supplied by for example broadcasting, communication or recording medium to a bitstream input terminal 220 and expanded by a moving picture expansion device 221 so as to be outputted at a picture output terminal 222. The picture signals, outputted at the picture output terminal 222, are entered via for example an edition unit, not shown, to a picture input terminal 240. The moving picture signals supplied to the picture input terminal 240 are compressed by a moving picture compression device 241 so as to be outputted at a bitstream output terminal 242. This bitstream is supplied by for example broadcasting, communication or recording medium to a bitstream input terminal 260 and expanded by a moving picture expansion device 261 so as to be outputted at a picture output terminal 262. The picture signals outputted at the picture output terminal 222 are sent to for example the edition unit for edition.
In the arrangement shown in FIG. 4, the moving picture compression device 201 and the moving picture expansion device 221 execute first compression/expansion, while the moving picture compression device 241 and the moving picture expansion device 261 execute second compression/expansion. The same holds for the third and following compression/expansion operations.
If the picture is repeatedly compressed/expanded by the above-described compression/expansion system, the picture quality is deteriorated each time the operations are repeated.
Thus it is said to be advisable to match the picture coding type at the time of compression for suppressing picture quality deterioration brought about by repeated compression/expansion. That is, picture quality deterioration is thought to be suppressed by using the same encoded picture type, that is the intra-coded picture or I-picture devoid of motion compensation, a forward predictive encoded picture or P-picture obtained on motion compensation from a temporally previous frame or a bidirectional prediction encoded picture or B-picture obtained on motion compensation from a temporally previous frame and a temporally succeeding frame, as that used for the moving picture compression device 201 and the moving picture compression device 241 of FIG. 4, from one frame to another.
It is also contemplated to set in store the motion vector and all parameters, such as quantized parameters for orthogonal transform. Specifically, the motion vector and all parameters, such as quantized parameters for orthogonal transform, in the moving picture compression device 201 in FIG. 4, are laid in store so as to be used in the moving picture compression device 241. If the motion vector and all parameters, such as quantized parameters for orthogonal transform, are laid in store in this manner, it becomes unnecessary to perform motion prediction again or re-quantization at the time of re-compression thus eliminating picture quality deterioration on repeated compression/expansion.
It has however been found experimentally that simply matching the picture coding type at the time of repeated compression/expansion still leads to significant deterioration such that picture quality is deteriorated each time the compression/expansion operations are repeated.
On the other hand, if the information such as the motion vector and all parameters, including quantized parameters for orthogonal transform, are laid in store, it becomes necessary to provide a separate recording medium for storage of the information, thus complicating the device structure. Moreover, the value laid in store are not necessarily optimum values, while the information volume is excessive for storage.
As described above, repeated compression/expansion of the picture by the moving picture compression/expansion method exploiting the conventional motion compensated inter-frame prediction leads to picture quality deterioration for each compression/expansion, while there has not been known a drastic method for overcoming this drawback.
It is therefore an object of the present invention to provide a moving picture compression device and a moving picture expansion device whereby it becomes possible to suppress picture quality deterioration otherwise produced on repeated compression/expansion.
In one aspect, the present invention provides a moving picture compression device including motion vector separating means for separating a motion vector multiplexed in a blanking portion of a moving picture signal and compression means for compressing the moving picture signal using the motion vector.
In another aspect, the present invention provides a moving picture expansion device including motion vector separating means for separating a motion vector supplied in a state of being appended to the compressed moving picture signal, expansion means for expanding the compressed moving picture signals using the separated motion vector and multiplexing means for multiplexing the separated motion vector in the blanking portion of the expanded moving picture signal.
According to the present invention, the motion vector used in the moving picture expansion device is multiplexed in a blanking portion of the picture signal and the moving picture expansion device uses the motion vector multiplexed in the blanking portion for compressing the picture, for evading the use of an inappropriate motion vector on the occasion of repeated compression/expansion.
More specifically, according to the present invention, the motion vector used in the moving picture expansion device is multiplexed in the blanking portion of the picture signals and outputted, while the moving picture compression device effects picture compression using the motion vector multiplexed in the blanking portion, so that there is no risk of an inappropriate motion vector being used for repeated compression/expansion, thus minimizing the signal deterioration otherwise caused by repeated compression/expansion.